In order to support the communication requirements of high-speed data w transmission applications of e.g. 25, 40 or 100 Gbps, optical links are used when links via an electrical wire have a too low bandwidth. When using such an optical link for transmitting a signal from a first electronic component to a second electronic component, the electrical signal to be transmitted is first converted into an optical signal, then the optical signal is coupled into an optical fiber via an optical transmitter and transmitted to the second electronic component via the optical fiber. At the second electronic component, the optical signal is received by means of an optical receiver and converted back into an electrical signal. This converted electrical signal is further processed in the second electronic component.
Optoelectronic components that perform the transduction between the optical and electrical signals are often referred to as transceivers, E/O engines or EOE engines.
As shown in FIG. 1, such a transceiver 100 which is used for converting an electric signal into an optical signal and vice versa, includes an electrically insulating substrate 102, for instance a printed circuit board (PCB) or a flexible printed circuit (FPC). The transceiver 100 further comprises a plurality of signal input lines 104 which are arranged as differential signal pairs and end in a front end contact region for electrically contacting further electronic components, for instance via another printed circuit board.
The other peripheral end of the signal input lines being opposed to the front end contact region 106 is connected to an electronic transmission unit 108 in a circuit connecting region 110. The electronic transmission unit 108 comprises driver circuitry for driving optical senders, for instance an array of vertical cavity surface emitting lasers (VCSEL) 112. The optical signal emitted by the laser diode array 112 is internally coupled to an optical conductor, for instance an optical fiber.
Furthermore, the transceiver unit 100 comprises a photo detector array 114, which comprises for instance photodiodes, such as so-called PIN diodes (p-intrinsic-n photodiodes). These PIN diodes are coupled to the optical fiber for receiving an optical signal and converting same into an electrical signal. The output of the PIN diodes 114 is coupled to an amplifier unit 116, which may comprise an array of transimpedance amplifiers (TIA) connected to respective outputs of the array of photodiodes 114.
A plurality of electrical signal output lines 120 are provided for connecting the front end contact region to the output terminals of the amplifier circuit 116. The signal output lines 120 are formed as differential lines analogously to the signal input lines 104.
A ground plane layer 118 is provided within the substrate 102 with a well-defined distance towards the input and output signal lines 104 and 120, respectively.
The laser diodes 112 and the driver circuit 108 as well as the PIN diodes 114 and the belonging amplifier unit 116 are all placed on the substrate 102 in a way that they are surrounded by the ground plane layer 118. As proposed in the international application PCT/EP2013/063694, the ground plane layer 118 is provided with openings 124 in the region of the front end contact region 106 in order to improve the signal quality at the transition point from the E/O engine 100 to e.g. a further printed circuit board (not shown in the figure).
Due to parasitic effects of the bond pads and ESD protection devices on the chip, the impedances of the front end of the transmission unit 108 as well as the transimpedance amplifier unit 116 exhibit a capacitive nature. Therefore, significant impedance drops in the area of the circuit connection region 110 of the electronic transmission unit 108 and of the amplifier unit 116 are observed. This impairs the performance of the optoelectronic unit and makes it difficult to meet the return loss specification.
The interconnection system preferably carries signals with minimal distortion. One type of distortion is called crosstalk. Crosstalk occurs when one signal creates an unwanted signal on another signal line. Generally, crosstalk is caused by electromagnetic coupling between signal lines and is therefore a particular problem for high-speed, high-density interconnection systems. Electromagnetic coupling increases when signal lines are closer together or when the signals they carry are of a higher frequency. Both of these conditions are present in a high-speed, high-density interconnection system.
FIG. 2 shows the result of a time-domain reflectometry (TDR) measurement at the circuit connecting region 110 obtained with 20 ps (20 to 80%) pulses without any impedance matching measures. It can be seen from FIG. 2 that a significant pulse is reflected back indicating an undesired disturbance in this region.